What is the remaining credit after 59 minutes of calls

Let's find the equation of the line:
let:
[tex]\begin{gathered} (x1,y1)=(33,15.05) \\ (x2,y2)=(51,12.35) \end{gathered}[/tex]So:
[tex]m=\frac{y2-y1}{x2-x1}=\frac{12.35-15.05}{51-33}=-\frac{2.7}{18}=-0.15[/tex]So:
[tex]\begin{gathered} y-y1=m(x-x1) \\ y-15.05=-0.15(x-33) \\ y-15.05=-0.15x+4.95 \\ y(x)=-0.15x+20 \end{gathered}[/tex]After 59 minutes:
[tex]\begin{gathered} x=59 \\ y(59)=-0.15(59)+20 \\ y(59)=11.15 \end{gathered}[/tex]